1. Field of the Invention
The present invention relates to a switch apparatus used for detecting a differential lock state in an automobile.
2. Description of the Related Art
Hitherto, there has been known a switch apparatus capable of detecting a locking state or an unlocking state of a differential apparatus, as illustrated in FIG. 3 (refer to Patent Document 1). FIG. 3 illustrates a cross section of a rear differential apparatus for an automobile.
The rear differential apparatus 101 illustrated in FIG. 3 includes a locking device which locks a differential mechanism 111 such that a lock member 105 supported by a differential case 103 moves in the axial direction and engages a side gear 107 with a dog clutch 109.
A differential lock state is detected by a switch apparatus 115 supported on the side of a differential carrier 113.
The switch apparatus 115 is equipped with a rod 117 for detection. A flange 119 of the rod 117 is capable of axially engaging with the periphery of a detecting plate 121 attached to the lock member 105.
For this reason, when the lock member 105 moves to lock the differential mechanism, the flange 119 engages with the periphery of the detecting plate 121 in the axial direction and draws the rod 117 out in the axial direction.
The movement of the rod 117 in the axial direction enables the switch apparatus 115 to detect the differential lock state.
The structure of the switch apparatus 115 is illustrated in a cross section in FIG. 4 and is the same as that described in Patent Document 2.
As illustrated in FIG. 4, the switch apparatus 115 has a contact housing portion 125 in a switch case 123 and a shaft hole 127 opened to outside at one end thereof.
The rod 117 is movably supported in the axial direction along the shaft hole 127 and biased by a return spring 129, which is disposed between the rod 117 and the switch case 123, toward the contact housing portion 125.
A movable contact 131 is disposed at the inner end of the rod 117 and opposes axially to a fixed contact 133 which is provided on the side of the switch case 123.
In accordance with the structure described above, when the differential mechanism is locked the rod 117 moves to bring the movable contact 131 into contact with the fixed contact 133, enabling the differential lock state to be detected.
Here, since the switch apparatus 115 is attached on the side of the differential carrier 113, as illustrated in FIG. 3, the distal end portion of the switch case 123 including the rod 117 is exposed to lubricating oil inside the differential carrier 113.
For that reason, the lubricating oil enters between the rod 117 and the shaft hole 127 and infiltrates into the contact housing portion 125 which is not liquid tight sealed. Hence sludge and contaminant included in the lubricating oil stick to the movable contact 131 and the fixed contact 133 to occur a contact failure due to roughness on the surface of the contact portion.
To solve the above problem, there may be adopted a configuration in which the shaft hole 127 and the contact housing portion 125 are liquid-tightly partitioned by a diaphragm of an elastic material such as rubber, and the rod 117 is arranged to press the diaphragm so as to transmit the action of the rod 117 to the contact housing portion 125, thereby enabling a movable portion which is supporting the movable contact 131 to move, or the like.
However, in the switch apparatus 115 for detecting a differential lock state as described above, since the flange 119 of the rod 117 abuts on and engages with the rotating detecting plate 121 in the axial direction, the rod 117 rotates in an interlocking manner with the detecting plate 121 generates a sliding contact between the rod 117 and the diaphragm, which may produce a problem in that the diaphragm loses its durability in a short period of time.
The problem caused when the rod 117 receives input in the rotation direction occurs not only in detecting a rotational object as described above, but also occurs when a rod receives a torsional force as in a case where a rod is designed to engage with a cam to detect its displacement, wherein the cam surface thrusts the rod.
[Patent Document 1] Japanese Patent Application Laid-Open No. 2006-29579
[Patent Document 2] Japanese Patent Application Laid-Open No. 09-265859
The problem to be solved is that the contact housing portion being made liquid tight loses its durability.